(1) Field of the Invention
The present invention relates to a method for tending to optimize both the noise emitted by an auxiliary rotor of a rotorcraft and also the performance of the rotorcraft, and it also relates to a rotorcraft applying the method.
The invention thus lies in the narrow technical field of tail fins for aircraft, and more particularly for rotorcraft.
(2) Description of Related Art
For example, a helicopter type rotorcraft may have a single main rotor that is driven mechanically by at least one engine. The main rotor then provides the helicopter with lift and propulsion.
The helicopter is also provided with an auxiliary tail rotor that performs an anti-torque function by exerting transverse thrust in order to compensate the yaw thrust created by rotating the main rotor. This torque is referred to below as “rotor torque” for convenience.
Furthermore, the auxiliary rotor enables the pilot to control the yaw movements of the helicopter by exerting transverse thrust that is positive or negative.
The auxiliary rotor may then for example be arranged on a tail fin carried by a tail boom of the aircraft. The term “fin” designates a streamlined body extending in elevation and that is substantially contained in a vertical plane. Nevertheless, the fin may be inclined relative to this vertical anteroposterior plane of symmetry. The term “vertical fin” is sometimes used.
An unducted auxiliary rotor is known, and for convenience is referred to below as a “conventional” auxiliary rotor. Conventionally, an unducted auxiliary rotor is mounted laterally at a top end of the tail fin.
Such an unducted auxiliary rotor is in widespread use. Nevertheless, it is possible to implement an auxiliary rotor that is ducted, as known under the trademark Fenestron®, for example.
A ducted auxiliary rotor comprises a rotor arranged in a duct provided through the tail fin of a helicopter. The axis of symmetry of the duct is substantially perpendicular to the vertical anteroposterior plane of symmetry of the helicopter.
Consequently, the streamlined shape of the vertical fin of the helicopter surrounds said duct and thus the auxiliary rotor. It should be observed that the streamlined structure is commonly referred to by the person skilled in the art as a fairing. Such a rotor is referred to for convenience below as a “ducted” rotor.
Independently of the ducted or unducted nature of the auxiliary rotor, the tail fin participates in controlling yaw movements. The fin generates transverse lift while the helicopter is in forward flight. The greater the forward speed of the helicopter, the greater this transverse lift.
A ducted or unducted auxiliary rotor thus makes it possible to control yaw movements of a rotorcraft. Nevertheless, an auxiliary rotor can generate a greater or smaller amount of noise depending on the stage of flight of the rotorcraft.
Document FR 2 338 845 refers to a helicopter having a rotor driven by an engine.
Document FR 2 338 845 then provides for controlling the helicopter in yaw by means of a fixed-pitch ducted propeller driven by the engine, with the thrust of that propeller being modulated by variable-pitch vanes situated in a duct of the propeller and upstream therefrom. The auxiliary rotor is thus a ducted rotor provided with a propeller and with vanes arranged in the duct of the ducted rotor.
Document EP 0 867 364 suggests reducing the noise emitted by a rotorcraft by reducing the speed of rotation of a main rotor, and by controlling accordingly an auxiliary rotor and a movable fin element. The pitch of the blades of the auxiliary rotor and the angle of attack of the movable fin element are determined on the basis of an air speed and of the torque exerted by the main rotor.
Document U.S. Pat. No. 6,290,171 provides for a device for controlling a hybrid anti-torque system for opposing the torque generated by a main rotor for providing a helicopter with lift and propulsion, and comprising:
an anti-torque auxiliary rotor that is controllable and that exerts anti-torque lateral thrust; and
at least one steering airfoil that is controllable and that generates anti-torque transverse thrust.
That device includes control means:
for controlling as a priority said airfoil so that it generates lift that is representative of at least a portion of a first control order, which portion is suitable for being executed by said airfoil; and
for controlling said auxiliary rotor so that the combined action of said airfoil and of said auxiliary rotor is representative of a yaw control order for the helicopter.
Document EP 1 547 919 describes a method and a device for reducing the vibration generated by the structure of a helicopter. That vibration results from the flow of air coming from a main rotor that provides the aircraft with lift and propulsion, and from the flow of air running along the fuselage. The method and the device then make use of a measurement of vibration in order to determine the angle of incidence of a tail fin in order to generate a force in opposition to the measured vibration.
Document EP 0 566 452 describes a helicopter having a single main lift and propulsion rotor, together with an anti-torque system. The anti-torque system comprises:
an anti-torque auxiliary rotor driven in rotation from engine means for said main rotor and exerting controllable anti-torque lateral thrust; and
at least one steering airfoil of controllable deflection for generating anti-torque transverse lift.
Under such circumstances, the helicopter includes means for automatically controlling the deflection angle of said steering airfoil as a function of the collective pitch angle of said main rotor and as a function of the forward speed of said helicopter.
Finally, Document DE 1 144 116 describes a fin carrying an auxiliary rotor and a control surface capable of being pivoted.
Also known is Document US 2012/104156.